connection.py

import copy
import errno
import io
import os
import socket
import threading
import weakref
from itertools import chain
from queue import Empty, Full, LifoQueue
from time import time
from typing import Optional
from urllib.parse import parse_qs, unquote, urlparse

from redis.backoff import NoBackoff
from redis.credentials import CredentialProvider, UsernamePasswordCredentialProvider
from redis.exceptions import (
    AuthenticationError,
    AuthenticationWrongNumberOfArgsError,
    BusyLoadingError,
    ChildDeadlockedError,
    ConnectionError,
    DataError,
    ExecAbortError,
    InvalidResponse,
    ModuleError,
    NoPermissionError,
    NoScriptError,
    ReadOnlyError,
    RedisError,
    ResponseError,
    TimeoutError,
)
from redis.retry import Retry
from redis.utils import CRYPTOGRAPHY_AVAILABLE, HIREDIS_AVAILABLE, str_if_bytes

try:
    import ssl

    ssl_available = True
except ImportError:
    ssl_available = False

NONBLOCKING_EXCEPTION_ERROR_NUMBERS = {BlockingIOError: errno.EWOULDBLOCK}

if ssl_available:
    if hasattr(ssl, "SSLWantReadError"):
        NONBLOCKING_EXCEPTION_ERROR_NUMBERS[ssl.SSLWantReadError] = 2
        NONBLOCKING_EXCEPTION_ERROR_NUMBERS[ssl.SSLWantWriteError] = 2
    else:
        NONBLOCKING_EXCEPTION_ERROR_NUMBERS[ssl.SSLError] = 2

NONBLOCKING_EXCEPTIONS = tuple(NONBLOCKING_EXCEPTION_ERROR_NUMBERS.keys())

if HIREDIS_AVAILABLE:
    import hiredis

SYM_STAR = b"*"
SYM_DOLLAR = b"$"
SYM_CRLF = b"\r\n"
SYM_EMPTY = b""

SERVER_CLOSED_CONNECTION_ERROR = "Connection closed by server."

SENTINEL = object()
MODULE_LOAD_ERROR = "Error loading the extension. Please check the server logs."
NO_SUCH_MODULE_ERROR = "Error unloading module: no such module with that name"
MODULE_UNLOAD_NOT_POSSIBLE_ERROR = "Error unloading module: operation not possible."
MODULE_EXPORTS_DATA_TYPES_ERROR = (
    "Error unloading module: the module "
    "exports one or more module-side data "
    "types, can't unload"
)
# user send an AUTH cmd to a server without authorization configured
NO_AUTH_SET_ERROR = {
    # Redis >= 6.0
    "AUTH <password> called without any password "
    "configured for the default user. Are you sure "
    "your configuration is correct?": AuthenticationError,
    # Redis < 6.0
    "Client sent AUTH, but no password is set": AuthenticationError,
}


class Encoder:
    "Encode strings to bytes-like and decode bytes-like to strings"

    def __init__(self, encoding, encoding_errors, decode_responses):
        self.encoding = encoding
        self.encoding_errors = encoding_errors
        self.decode_responses = decode_responses

    def encode(self, value):
        "Return a bytestring or bytes-like representation of the value"
        if isinstance(value, (bytes, memoryview)):
            return value
        elif isinstance(value, bool):
            # special case bool since it is a subclass of int
            raise DataError(
                "Invalid input of type: 'bool'. Convert to a "
                "bytes, string, int or float first."
            )
        elif isinstance(value, (int, float)):
            value = repr(value).encode()
        elif not isinstance(value, str):
            # a value we don't know how to deal with. throw an error
            typename = type(value).__name__
            raise DataError(
                f"Invalid input of type: '{typename}'. "
                f"Convert to a bytes, string, int or float first."
            )
        if isinstance(value, str):
            value = value.encode(self.encoding, self.encoding_errors)
        return value

    def decode(self, value, force=False):
        "Return a unicode string from the bytes-like representation"
        if self.decode_responses or force:
            if isinstance(value, memoryview):
                value = value.tobytes()
            if isinstance(value, bytes):
                value = value.decode(self.encoding, self.encoding_errors)
        return value


class BaseParser:
    EXCEPTION_CLASSES = {
        "ERR": {
            "max number of clients reached": ConnectionError,
            "invalid password": AuthenticationError,
            # some Redis server versions report invalid command syntax
            # in lowercase
            "wrong number of arguments "
            "for 'auth' command": AuthenticationWrongNumberOfArgsError,
            # some Redis server versions report invalid command syntax
            # in uppercase
            "wrong number of arguments "
            "for 'AUTH' command": AuthenticationWrongNumberOfArgsError,
            MODULE_LOAD_ERROR: ModuleError,
            MODULE_EXPORTS_DATA_TYPES_ERROR: ModuleError,
            NO_SUCH_MODULE_ERROR: ModuleError,
            MODULE_UNLOAD_NOT_POSSIBLE_ERROR: ModuleError,
            **NO_AUTH_SET_ERROR,
        },
        "WRONGPASS": AuthenticationError,
        "EXECABORT": ExecAbortError,
        "LOADING": BusyLoadingError,
        "NOSCRIPT": NoScriptError,
        "READONLY": ReadOnlyError,
        "NOAUTH": AuthenticationError,
        "NOPERM": NoPermissionError,
    }

    def parse_error(self, response):
        "Parse an error response"
        error_code = response.split(" ")[0]
        if error_code in self.EXCEPTION_CLASSES:
            response = response[len(error_code) + 1 :]
            exception_class = self.EXCEPTION_CLASSES[error_code]
            if isinstance(exception_class, dict):
                exception_class = exception_class.get(response, ResponseError)
            return exception_class(response)
        return ResponseError(response)


class SocketBuffer:
    def __init__(self, socket, socket_read_size, socket_timeout):
        self._sock = socket
        self.socket_read_size = socket_read_size
        self.socket_timeout = socket_timeout
        self._buffer = io.BytesIO()
        # number of bytes written to the buffer from the socket
        self.bytes_written = 0
        # number of bytes read from the buffer
        self.bytes_read = 0

    @property
    def length(self):
        return self.bytes_written - self.bytes_read

    def _read_from_socket(self, length=None, timeout=SENTINEL, raise_on_timeout=True):
        sock = self._sock
        socket_read_size = self.socket_read_size
        buf = self._buffer
        buf.seek(self.bytes_written)
        marker = 0
        custom_timeout = timeout is not SENTINEL

        try:
            if custom_timeout:
                sock.settimeout(timeout)
            while True:
                data = self._sock.recv(socket_read_size)
                # an empty string indicates the server shutdown the socket
                if isinstance(data, bytes) and len(data) == 0:
                    raise ConnectionError(SERVER_CLOSED_CONNECTION_ERROR)
                buf.write(data)
                data_length = len(data)
                self.bytes_written += data_length
                marker += data_length

                if length is not None and length > marker:
                    continue
                return True
        except socket.timeout:
            if raise_on_timeout:
                raise TimeoutError("Timeout reading from socket")
            return False
        except NONBLOCKING_EXCEPTIONS as ex:
            # if we're in nonblocking mode and the recv raises a
            # blocking error, simply return False indicating that
            # there's no data to be read. otherwise raise the
            # original exception.
            allowed = NONBLOCKING_EXCEPTION_ERROR_NUMBERS.get(ex.__class__, -1)
            if not raise_on_timeout and ex.errno == allowed:
                return False
            raise ConnectionError(f"Error while reading from socket: {ex.args}")
        finally:
            if custom_timeout:
                sock.settimeout(self.socket_timeout)

    def can_read(self, timeout):
        return bool(self.length) or self._read_from_socket(
            timeout=timeout, raise_on_timeout=False
        )

    def read(self, length):
        length = length + 2  # make sure to read the \r\n terminator
        # make sure we've read enough data from the socket
        if length > self.length:
            self._read_from_socket(length - self.length)

        self._buffer.seek(self.bytes_read)
        data = self._buffer.read(length)
        self.bytes_read += len(data)

        # purge the buffer when we've consumed it all so it doesn't
        # grow forever
        if self.bytes_read == self.bytes_written:
            self.purge()

        return data[:-2]

    def readline(self):
        buf = self._buffer
        buf.seek(self.bytes_read)
        data = buf.readline()
        while not data.endswith(SYM_CRLF):
            # there's more data in the socket that we need
            self._read_from_socket()
            buf.seek(self.bytes_read)
            data = buf.readline()

        self.bytes_read += len(data)

        # purge the buffer when we've consumed it all so it doesn't
        # grow forever
        if self.bytes_read == self.bytes_written:
            self.purge()

        return data[:-2]

    def purge(self):
        self._buffer.seek(0)
        self._buffer.truncate()
        self.bytes_written = 0
        self.bytes_read = 0

    def close(self):
        try:
            self.purge()
            self._buffer.close()
        except Exception:
            # issue #633 suggests the purge/close somehow raised a
            # BadFileDescriptor error. Perhaps the client ran out of
            # memory or something else? It's probably OK to ignore
            # any error being raised from purge/close since we're
            # removing the reference to the instance below.
            pass
        self._buffer = None
        self._sock = None


class PythonParser(BaseParser):
    "Plain Python parsing class"

    def __init__(self, socket_read_size):
        self.socket_read_size = socket_read_size
        self.encoder = None
        self._sock = None
        self._buffer = None

    def __del__(self):
        try:
            self.on_disconnect()
        except Exception:
            pass

    def on_connect(self, connection):
        "Called when the socket connects"
        self._sock = connection._sock
        self._buffer = SocketBuffer(
            self._sock, self.socket_read_size, connection.socket_timeout
        )
        self.encoder = connection.encoder

    def on_disconnect(self):
        "Called when the socket disconnects"
        self._sock = None
        if self._buffer is not None:
            self._buffer.close()
            self._buffer = None
        self.encoder = None

    def can_read(self, timeout):
        return self._buffer and self._buffer.can_read(timeout)

    def read_response(self, disable_decoding=False):
        raw = self._buffer.readline()
        if not raw:
            raise ConnectionError(SERVER_CLOSED_CONNECTION_ERROR)

        byte, response = raw[:1], raw[1:]

        if byte not in (b"-", b"+", b":", b"$", b"*"):
            raise InvalidResponse(f"Protocol Error: {raw!r}")

        # server returned an error
        if byte == b"-":
            response = response.decode("utf-8", errors="replace")
            error = self.parse_error(response)
            # if the error is a ConnectionError, raise immediately so the user
            # is notified
            if isinstance(error, ConnectionError):
                raise error
            # otherwise, we're dealing with a ResponseError that might belong
            # inside a pipeline response. the connection's read_response()
            # and/or the pipeline's execute() will raise this error if
            # necessary, so just return the exception instance here.
            return error
        # single value
        elif byte == b"+":
            pass
        # int value
        elif byte == b":":
            response = int(response)
        # bulk response
        elif byte == b"$":
            length = int(response)
            if length == -1:
                return None
            response = self._buffer.read(length)
        # multi-bulk response
        elif byte == b"*":
            length = int(response)
            if length == -1:
                return None
            response = [
                self.read_response(disable_decoding=disable_decoding)
                for i in range(length)
            ]
        if isinstance(response, bytes) and disable_decoding is False:
            response = self.encoder.decode(response)
        return response


class HiredisParser(BaseParser):
    "Parser class for connections using Hiredis"

    def __init__(self, socket_read_size):
        if not HIREDIS_AVAILABLE:
            raise RedisError("Hiredis is not installed")
        self.socket_read_size = socket_read_size
        self._buffer = bytearray(socket_read_size)

    def __del__(self):
        try:
            self.on_disconnect()
        except Exception:
            pass

    def on_connect(self, connection, **kwargs):
        self._sock = connection._sock
        self._socket_timeout = connection.socket_timeout
        kwargs = {
            "protocolError": InvalidResponse,
            "replyError": self.parse_error,
            "errors": connection.encoder.encoding_errors,
        }

        if connection.encoder.decode_responses:
            kwargs["encoding"] = connection.encoder.encoding
        self._reader = hiredis.Reader(**kwargs)
        self._next_response = False

    def on_disconnect(self):
        self._sock = None
        self._reader = None
        self._next_response = False

    def can_read(self, timeout):
        if not self._reader:
            raise ConnectionError(SERVER_CLOSED_CONNECTION_ERROR)

        if self._next_response is False:
            self._next_response = self._reader.gets()
            if self._next_response is False:
                return self.read_from_socket(timeout=timeout, raise_on_timeout=False)
        return True

    def read_from_socket(self, timeout=SENTINEL, raise_on_timeout=True):
        sock = self._sock
        custom_timeout = timeout is not SENTINEL
        try:
            if custom_timeout:
                sock.settimeout(timeout)
            bufflen = self._sock.recv_into(self._buffer)
            if bufflen == 0:
                raise ConnectionError(SERVER_CLOSED_CONNECTION_ERROR)
            self._reader.feed(self._buffer, 0, bufflen)
            # data was read from the socket and added to the buffer.
            # return True to indicate that data was read.
            return True
        except socket.timeout:
            if raise_on_timeout:
                raise TimeoutError("Timeout reading from socket")
            return False
        except NONBLOCKING_EXCEPTIONS as ex:
            # if we're in nonblocking mode and the recv raises a
            # blocking error, simply return False indicating that
            # there's no data to be read. otherwise raise the
            # original exception.
            allowed = NONBLOCKING_EXCEPTION_ERROR_NUMBERS.get(ex.__class__, -1)
            if not raise_on_timeout and ex.errno == allowed:
                return False
            raise ConnectionError(f"Error while reading from socket: {ex.args}")
        finally:
            if custom_timeout:
                sock.settimeout(self._socket_timeout)

    def read_response(self, disable_decoding=False):
        if not self._reader:
            raise ConnectionError(SERVER_CLOSED_CONNECTION_ERROR)

        # _next_response might be cached from a can_read() call
        if self._next_response is not False:
            response = self._next_response
            self._next_response = False
            return response

        if disable_decoding:
            response = self._reader.gets(False)
        else:
            response = self._reader.gets()

        while response is False:
            self.read_from_socket()
            if disable_decoding:
                response = self._reader.gets(False)
            else:
                response = self._reader.gets()
        # if the response is a ConnectionError or the response is a list and
        # the first item is a ConnectionError, raise it as something bad
        # happened
        if isinstance(response, ConnectionError):
            raise response
        elif (
            isinstance(response, list)
            and response
            and isinstance(response[0], ConnectionError)
        ):
            raise response[0]
        return response


if HIREDIS_AVAILABLE:
    DefaultParser = HiredisParser
else:
    DefaultParser = PythonParser


class Connection:
    "Manages TCP communication to and from a Redis server"

    def __init__(
        self,
        host="localhost",
        port=6379,
        db=0,
        password=None,
        socket_timeout=None,
        socket_connect_timeout=None,
        socket_keepalive=False,
        socket_keepalive_options=None,
        socket_type=0,
        retry_on_timeout=False,
        retry_on_error=SENTINEL,
        encoding="utf-8",
        encoding_errors="strict",
        decode_responses=False,
        parser_class=DefaultParser,
        socket_read_size=65536,
        health_check_interval=0,
        client_name=None,
        username=None,
        retry=None,
        redis_connect_func=None,
        credential_provider: Optional[CredentialProvider] = None,
    ):
        """
        Initialize a new Connection.
        To specify a retry policy for specific errors, first set
        `retry_on_error` to a list of the error/s to retry on, then set
        `retry` to a valid `Retry` object.
        To retry on TimeoutError, `retry_on_timeout` can also be set to `True`.
        """
        if (username or password) and credential_provider is not None:
            raise DataError(
                "'username' and 'password' cannot be passed along with 'credential_"
                "provider'. Please provide only one of the following arguments: \n"
                "1. 'password' and (optional) 'username'\n"
                "2. 'credential_provider'"
            )
        self.pid = os.getpid()
        self.host = host
        self.port = int(port)
        self.db = db
        self.client_name = client_name
        self.credential_provider = credential_provider
        self.password = password
        self.username = username
        self.socket_timeout = socket_timeout
        self.socket_connect_timeout = socket_connect_timeout or socket_timeout
        self.socket_keepalive = socket_keepalive
        self.socket_keepalive_options = socket_keepalive_options or {}
        self.socket_type = socket_type
        self.retry_on_timeout = retry_on_timeout
        if retry_on_error is SENTINEL:
            retry_on_error = []
        if retry_on_timeout:
            # Add TimeoutError to the errors list to retry on
            retry_on_error.append(TimeoutError)
        self.retry_on_error = retry_on_error
        if retry or retry_on_error:
            if retry is None:
                self.retry = Retry(NoBackoff(), 1)
            else:
                # deep-copy the Retry object as it is mutable
                self.retry = copy.deepcopy(retry)
            # Update the retry's supported errors with the specified errors
            self.retry.update_supported_errors(retry_on_error)
        else:
            self.retry = Retry(NoBackoff(), 0)
        self.health_check_interval = health_check_interval
        self.next_health_check = 0
        self.redis_connect_func = redis_connect_func
        self.encoder = Encoder(encoding, encoding_errors, decode_responses)
        self._sock = None
        self._socket_read_size = socket_read_size
        self.set_parser(parser_class)
        self._connect_callbacks = []
        self._buffer_cutoff = 6000

    def __repr__(self):
        repr_args = ",".join([f"{k}={v}" for k, v in self.repr_pieces()])
        return f"{self.__class__.__name__}<{repr_args}>"

    def repr_pieces(self):
        pieces = [("host", self.host), ("port", self.port), ("db", self.db)]
        if self.client_name:
            pieces.append(("client_name", self.client_name))
        return pieces

    def __del__(self):
        try:
            self.disconnect()
        except Exception:
            pass

    def register_connect_callback(self, callback):
        self._connect_callbacks.append(weakref.WeakMethod(callback))

    def clear_connect_callbacks(self):
        self._connect_callbacks = []

    def set_parser(self, parser_class):
        """
        Creates a new instance of parser_class with socket size:
        _socket_read_size and assigns it to the parser for the connection
        :param parser_class: The required parser class
        """
        self._parser = parser_class(socket_read_size=self._socket_read_size)

    def connect(self):
        "Connects to the Redis server if not already connected"
        if self._sock:
            return
        try:
            sock = self.retry.call_with_retry(
                lambda: self._connect(), lambda error: self.disconnect(error)
            )
        except socket.timeout:
            raise TimeoutError("Timeout connecting to server")
        except OSError as e:
            raise ConnectionError(self._error_message(e))

        self._sock = sock
        try:
            if self.redis_connect_func is None:
                # Use the default on_connect function
                self.on_connect()
            else:
                # Use the passed function redis_connect_func
                self.redis_connect_func(self)
        except RedisError:
            # clean up after any error in on_connect
            self.disconnect()
            raise

        # run any user callbacks. right now the only internal callback
        # is for pubsub channel/pattern resubscription
        for ref in self._connect_callbacks:
            callback = ref()
            if callback:
                callback(self)

    def _connect(self):
        "Create a TCP socket connection"
        # we want to mimic what socket.create_connection does to support
        # ipv4/ipv6, but we want to set options prior to calling
        # socket.connect()
        err = None
        for res in socket.getaddrinfo(
            self.host, self.port, self.socket_type, socket.SOCK_STREAM
        ):
            family, socktype, proto, canonname, socket_address = res
            sock = None
            try:
                sock = socket.socket(family, socktype, proto)
                # TCP_NODELAY
                sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)

                # TCP_KEEPALIVE
                if self.socket_keepalive:
                    sock.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
                    for k, v in self.socket_keepalive_options.items():
                        sock.setsockopt(socket.IPPROTO_TCP, k, v)

                # set the socket_connect_timeout before we connect
                sock.settimeout(self.socket_connect_timeout)

                # connect
                sock.connect(socket_address)

                # set the socket_timeout now that we're connected
                sock.settimeout(self.socket_timeout)
                return sock

            except OSError as _:
                err = _
                if sock is not None:
                    sock.close()

        if err is not None:
            raise err
        raise OSError("socket.getaddrinfo returned an empty list")

    def _error_message(self, exception):
        # args for socket.error can either be (errno, "message")
        # or just "message"
        if len(exception.args) == 1:
            try:
                return f"Error connecting to {self.host}:{self.port}. \
                        {exception.args[0]}."
            except AttributeError:
                return f"Connection Error: {exception.args[0]}"
        else:
            try:
                return (
                    f"Error {exception.args[0]} connecting to "
                    f"{self.host}:{self.port}. {exception.args[1]}."
                )
            except AttributeError:
                return f"Connection Error: {exception.args[0]}"

    def on_connect(self):
        "Initialize the connection, authenticate and select a database"
        self._parser.on_connect(self)

        # if credential provider or username and/or password are set, authenticate
        if self.credential_provider or (self.username or self.password):
            cred_provider = (
                self.credential_provider
                or UsernamePasswordCredentialProvider(self.username, self.password)
            )
            auth_args = cred_provider.get_credentials()
            # avoid checking health here -- PING will fail if we try
            # to check the health prior to the AUTH
            self.send_command("AUTH", *auth_args, check_health=False)

            try:
                auth_response = self.read_response()
            except AuthenticationWrongNumberOfArgsError:
                # a username and password were specified but the Redis
                # server seems to be < 6.0.0 which expects a single password
                # arg. retry auth with just the password.
                # https://github.com/andymccurdy/redis-py/issues/1274
                self.send_command("AUTH", auth_args[-1], check_health=False)
                auth_response = self.read_response()

            if str_if_bytes(auth_response) != "OK":
                raise AuthenticationError("Invalid Username or Password")

        # if a client_name is given, set it
        if self.client_name:
            self.send_command("CLIENT", "SETNAME", self.client_name)
            if str_if_bytes(self.read_response()) != "OK":
                raise ConnectionError("Error setting client name")

        # if a database is specified, switch to it
        if self.db:
            self.send_command("SELECT", self.db)
            if str_if_bytes(self.read_response()) != "OK":
                raise ConnectionError("Invalid Database")

    def disconnect(self, *args):
        "Disconnects from the Redis server"
        self._parser.on_disconnect()
        if self._sock is None:
            return

        if os.getpid() == self.pid:
            try:
                self._sock.shutdown(socket.SHUT_RDWR)
            except OSError:
                pass

        try:
            self._sock.close()
        except OSError:
            pass
        self._sock = None

    def _send_ping(self):
        """Send PING, expect PONG in return"""
        self.send_command("PING", check_health=False)
        if str_if_bytes(self.read_response()) != "PONG":
            raise ConnectionError("Bad response from PING health check")

    def _ping_failed(self, error):
        """Function to call when PING fails"""
        self.disconnect()

    def check_health(self):
        """Check the health of the connection with a PING/PONG"""
        if self.health_check_interval and time() > self.next_health_check:
            self.retry.call_with_retry(self._send_ping, self._ping_failed)

    def send_packed_command(self, command, check_health=True):
        """Send an already packed command to the Redis server"""
        if not self._sock:
            self.connect()
        # guard against health check recursion
        if check_health:
            self.check_health()
        try:
            if isinstance(command, str):
                command = [command]
            for item in command:
                self._sock.sendall(item)
        except socket.timeout:
            self.disconnect()
            raise TimeoutError("Timeout writing to socket")
        except OSError as e:
            self.disconnect()
            if len(e.args) == 1:
                errno, errmsg = "UNKNOWN", e.args[0]
            else:
                errno = e.args[0]
                errmsg = e.args[1]
            raise ConnectionError(f"Error {errno} while writing to socket. {errmsg}.")
        except BaseException:
            # The send_packed_command api does not support re-trying a partially
            # sent message, so there is no point in keeping the connection open.
            # An unknown number of bytes has been sent and the connection is therefore
            # unusable.
            self.disconnect()
            raise

    def send_command(self, *args, **kwargs):
        """Pack and send a command to the Redis server"""
        self.send_packed_command(
            self.pack_command(*args), check_health=kwargs.get("check_health", True)
        )

    def can_read(self, timeout=0):
        """Poll the socket to see if there's data that can be read."""
        sock = self._sock
        if not sock:
            self.connect()
        try:
            return self._parser.can_read(timeout)
        except OSError as e:
            self.disconnect()
            raise ConnectionError(
                f"Error while reading from {self.host}:{self.port}: {e.args}"
            )

    def read_response(self, disable_decoding=False, disconnect_on_error: bool = True):
        """Read the response from a previously sent command"""
        try:
            hosterr = f"{self.host}:{self.port}"
        except AttributeError:
            hosterr = "connection"

        try:
            response = self._parser.read_response(disable_decoding=disable_decoding)
        except socket.timeout:
            if disconnect_on_error:
                self.disconnect()
            raise TimeoutError(f"Timeout reading from {hosterr}")
        except OSError as e:
            if disconnect_on_error:
                self.disconnect()
            raise ConnectionError(f"Error while reading from {hosterr}" f" : {e.args}")
        except BaseException:
            # Also by default close in case of BaseException.  A lot of code
            # relies on this behaviour when doing Command/Response pairs.
            # See #1128.
            if disconnect_on_error:
                self.disconnect()
            raise

        if self.health_check_interval:
            self.next_health_check = time() + self.health_check_interval

        if isinstance(response, ResponseError):
            raise response
        return response

    def pack_command(self, *args):
        """Pack a series of arguments into the Redis protocol"""
        output = []
        # the client might have included 1 or more literal arguments in
        # the command name, e.g., 'CONFIG GET'. The Redis server expects these
        # arguments to be sent separately, so split the first argument
        # manually. These arguments should be bytestrings so that they are
        # not encoded.
        if isinstance(args[0], str):
            args = tuple(args[0].encode().split()) + args[1:]
        elif b" " in args[0]:
            args = tuple(args[0].split()) + args[1:]

        buff = SYM_EMPTY.join((SYM_STAR, str(len(args)).encode(), SYM_CRLF))

        buffer_cutoff = self._buffer_cutoff
        for arg in map(self.encoder.encode, args):
            # to avoid large string mallocs, chunk the command into the
            # output list if we're sending large values or memoryviews
            arg_length = len(arg)
            if (
                len(buff) > buffer_cutoff
                or arg_length > buffer_cutoff
                or isinstance(arg, memoryview)
            ):
                buff = SYM_EMPTY.join(
                    (buff, SYM_DOLLAR, str(arg_length).encode(), SYM_CRLF)
                )
                output.append(buff)
                output.append(arg)
                buff = SYM_CRLF
            else:
                buff = SYM_EMPTY.join(
                    (
                        buff,
                        SYM_DOLLAR,
                        str(arg_length).encode(),
                        SYM_CRLF,
                        arg,
                        SYM_CRLF,
                    )
                )
        output.append(buff)
        return output

    def pack_commands(self, commands):
        """Pack multiple commands into the Redis protocol"""
        output = []
        pieces = []
        buffer_length = 0
        buffer_cutoff = self._buffer_cutoff

        for cmd in commands:
            for chunk in self.pack_command(*cmd):
                chunklen = len(chunk)
                if (
                    buffer_length > buffer_cutoff
                    or chunklen > buffer_cutoff
                    or isinstance(chunk, memoryview)
                ):
                    output.append(SYM_EMPTY.join(pieces))
                    buffer_length = 0
                    pieces = []

                if chunklen > buffer_cutoff or isinstance(chunk, memoryview):
                    output.append(chunk)
                else:
                    pieces.append(chunk)
                    buffer_length += chunklen

        if pieces:
            output.append(SYM_EMPTY.join(pieces))
        return output


class SSLConnection(Connection):
    """Manages SSL connections to and from the Redis server(s).
    This class extends the Connection class, adding SSL functionality, and making
    use of ssl.SSLContext (https://docs.python.org/3/library/ssl.html#ssl.SSLContext)
    """  # noqa

    def __init__(
        self,
        ssl_keyfile=None,
        ssl_certfile=None,
        ssl_cert_reqs="required",
        ssl_ca_certs=None,
        ssl_ca_data=None,
        ssl_check_hostname=False,
        ssl_ca_path=None,
        ssl_password=None,
        ssl_validate_ocsp=False,
        ssl_validate_ocsp_stapled=False,
        ssl_ocsp_context=None,
        ssl_ocsp_expected_cert=None,
        **kwargs,
    ):
        """Constructor

        Args:
            ssl_keyfile: Path to an ssl private key. Defaults to None.
            ssl_certfile: Path to an ssl certificate. Defaults to None.
            ssl_cert_reqs: The string value for the SSLContext.verify_mode (none, optional, required). Defaults to "required".
            ssl_ca_certs: The path to a file of concatenated CA certificates in PEM format. Defaults to None.
            ssl_ca_data: Either an ASCII string of one or more PEM-encoded certificates or a bytes-like object of DER-encoded certificates.
            ssl_check_hostname: If set, match the hostname during the SSL handshake. Defaults to False.
            ssl_ca_path: The path to a directory containing several CA certificates in PEM format. Defaults to None.
            ssl_password: Password for unlocking an encrypted private key. Defaults to None.

            ssl_validate_ocsp: If set, perform a full ocsp validation (i.e not a stapled verification)
            ssl_validate_ocsp_stapled: If set, perform a validation on a stapled ocsp response
            ssl_ocsp_context: A fully initialized OpenSSL.SSL.Context object to be used in verifying the ssl_ocsp_expected_cert
            ssl_ocsp_expected_cert: A PEM armoured string containing the expected certificate to be returned from the ocsp verification service.

        Raises:
            RedisError
        """  # noqa
        if not ssl_available:
            raise RedisError("Python wasn't built with SSL support")

        super().__init__(**kwargs)

        self.keyfile = ssl_keyfile
        self.certfile = ssl_certfile
        if ssl_cert_reqs is None:
            ssl_cert_reqs = ssl.CERT_NONE
        elif isinstance(ssl_cert_reqs, str):
            CERT_REQS = {
                "none": ssl.CERT_NONE,
                "optional": ssl.CERT_OPTIONAL,
                "required": ssl.CERT_REQUIRED,
            }
            if ssl_cert_reqs not in CERT_REQS:
                raise RedisError(
                    f"Invalid SSL Certificate Requirements Flag: {ssl_cert_reqs}"
                )
            ssl_cert_reqs = CERT_REQS[ssl_cert_reqs]
        self.cert_reqs = ssl_cert_reqs
        self.ca_certs = ssl_ca_certs
        self.ca_data = ssl_ca_data
        self.ca_path = ssl_ca_path
        self.check_hostname = ssl_check_hostname
        self.certificate_password = ssl_password
        self.ssl_validate_ocsp = ssl_validate_ocsp
        self.ssl_validate_ocsp_stapled = ssl_validate_ocsp_stapled
        self.ssl_ocsp_context = ssl_ocsp_context
        self.ssl_ocsp_expected_cert = ssl_ocsp_expected_cert

    def _connect(self):
        "Wrap the socket with SSL support"
        sock = super()._connect()
        context = ssl.create_default_context()
        context.check_hostname = self.check_hostname
        context.verify_mode = self.cert_reqs
        if self.certfile or self.keyfile:
            context.load_cert_chain(
                certfile=self.certfile,
                keyfile=self.keyfile,
                password=self.certificate_password,
            )
        if (
            self.ca_certs is not None
            or self.ca_path is not None
            or self.ca_data is not None
        ):
            context.load_verify_locations(
                cafile=self.ca_certs, capath=self.ca_path, cadata=self.ca_data
            )
        sslsock = context.wrap_socket(sock, server_hostname=self.host)
        if self.ssl_validate_ocsp is True and CRYPTOGRAPHY_AVAILABLE is False:
            raise RedisError("cryptography is not installed.")

        if self.ssl_validate_ocsp_stapled and self.ssl_validate_ocsp:
            raise RedisError(
                "Either an OCSP staple or pure OCSP connection must be validated "
                "- not both."
            )

        # validation for the stapled case
        if self.ssl_validate_ocsp_stapled:
            import OpenSSL

            from .ocsp import ocsp_staple_verifier

            # if a context is provided use it - otherwise, a basic context
            if self.ssl_ocsp_context is None:
                staple_ctx = OpenSSL.SSL.Context(OpenSSL.SSL.SSLv23_METHOD)
                staple_ctx.use_certificate_file(self.certfile)
                staple_ctx.use_privatekey_file(self.keyfile)
            else:
                staple_ctx = self.ssl_ocsp_context

            staple_ctx.set_ocsp_client_callback(
                ocsp_staple_verifier, self.ssl_ocsp_expected_cert
            )

            #  need another socket
            con = OpenSSL.SSL.Connection(staple_ctx, socket.socket())
            con.request_ocsp()
            con.connect((self.host, self.port))
            con.do_handshake()
            con.shutdown()
            return sslsock

        # pure ocsp validation
        if self.ssl_validate_ocsp is True and CRYPTOGRAPHY_AVAILABLE:
            from .ocsp import OCSPVerifier

            o = OCSPVerifier(sslsock, self.host, self.port, self.ca_certs)
            if o.is_valid():
                return sslsock
            else:
                raise ConnectionError("ocsp validation error")
        return sslsock


class UnixDomainSocketConnection(Connection):
    def __init__(
        self,
        path="",
        db=0,
        username=None,
        password=None,
        socket_timeout=None,
        encoding="utf-8",
        encoding_errors="strict",
        decode_responses=False,
        retry_on_timeout=False,
        retry_on_error=SENTINEL,
        parser_class=DefaultParser,
        socket_read_size=65536,
        health_check_interval=0,
        client_name=None,
        retry=None,
        redis_connect_func=None,
        credential_provider: Optional[CredentialProvider] = None,
    ):
        """
        Initialize a new UnixDomainSocketConnection.
        To specify a retry policy for specific errors, first set
        `retry_on_error` to a list of the error/s to retry on, then set
        `retry` to a valid `Retry` object.
        To retry on TimeoutError, `retry_on_timeout` can also be set to `True`.
        """
        if (username or password) and credential_provider is not None:
            raise DataError(
                "'username' and 'password' cannot be passed along with 'credential_"
                "provider'. Please provide only one of the following arguments: \n"
                "1. 'password' and (optional) 'username'\n"
                "2. 'credential_provider'"
            )
        self.pid = os.getpid()
        self.path = path
        self.db = db
        self.client_name = client_name
        self.credential_provider = credential_provider
        self.password = password
        self.username = username
        self.socket_timeout = socket_timeout
        self.retry_on_timeout = retry_on_timeout
        if retry_on_error is SENTINEL:
            retry_on_error = []
        if retry_on_timeout:
            # Add TimeoutError to the errors list to retry on
            retry_on_error.append(TimeoutError)
        self.retry_on_error = retry_on_error
        if self.retry_on_error:
            if retry is None:
                self.retry = Retry(NoBackoff(), 1)
            else:
                # deep-copy the Retry object as it is mutable
                self.retry = copy.deepcopy(retry)
            # Update the retry's supported errors with the specified errors
            self.retry.update_supported_errors(retry_on_error)
        else:
            self.retry = Retry(NoBackoff(), 0)
        self.health_check_interval = health_check_interval
        self.next_health_check = 0
        self.redis_connect_func = redis_connect_func
        self.encoder = Encoder(encoding, encoding_errors, decode_responses)
        self._sock = None
        self._socket_read_size = socket_read_size
        self.set_parser(parser_class)
        self._connect_callbacks = []
        self._buffer_cutoff = 6000

    def repr_pieces(self):
        pieces = [("path", self.path), ("db", self.db)]
        if self.client_name:
            pieces.append(("client_name", self.client_name))
        return pieces

    def _connect(self):
        "Create a Unix domain socket connection"
        sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
        sock.settimeout(self.socket_timeout)
        sock.connect(self.path)
        return sock

    def _error_message(self, exception):
        # args for socket.error can either be (errno, "message")
        # or just "message"
        if len(exception.args) == 1:
            return f"Error connecting to unix socket: {self.path}. {exception.args[0]}."
        else:
            return (
                f"Error {exception.args[0]} connecting to unix socket: "
                f"{self.path}. {exception.args[1]}."
            )


FALSE_STRINGS = ("0", "F", "FALSE", "N", "NO")


def to_bool(value):
    if value is None or value == "":
        return None
    if isinstance(value, str) and value.upper() in FALSE_STRINGS:
        return False
    return bool(value)


URL_QUERY_ARGUMENT_PARSERS = {
    "db": int,
    "socket_timeout": float,
    "socket_connect_timeout": float,
    "socket_keepalive": to_bool,
    "retry_on_timeout": to_bool,
    "retry_on_error": list,
    "max_connections": int,
    "health_check_interval": int,
    "ssl_check_hostname": to_bool,
}


def parse_url(url):
    if not (
        url.startswith("redis://")
        or url.startswith("rediss://")
        or url.startswith("unix://")
    ):
        raise ValueError(
            "Redis URL must specify one of the following "
            "schemes (redis://, rediss://, unix://)"
        )

    url = urlparse(url)
    kwargs = {}

    for name, value in parse_qs(url.query).items():
        if value and len(value) > 0:
            value = unquote(value[0])
            parser = URL_QUERY_ARGUMENT_PARSERS.get(name)
            if parser:
                try:
                    kwargs[name] = parser(value)
                except (TypeError, ValueError):
                    raise ValueError(f"Invalid value for `{name}` in connection URL.")
            else:
                kwargs[name] = value

    if url.username:
        kwargs["username"] = unquote(url.username)
    if url.password:
        kwargs["password"] = unquote(url.password)

    # We only support redis://, rediss:// and unix:// schemes.
    if url.scheme == "unix":
        if url.path:
            kwargs["path"] = unquote(url.path)
        kwargs["connection_class"] = UnixDomainSocketConnection

    else:  # implied:  url.scheme in ("redis", "rediss"):
        if url.hostname:
            kwargs["host"] = unquote(url.hostname)
        if url.port:
            kwargs["port"] = int(url.port)

        # If there's a path argument, use it as the db argument if a
        # querystring value wasn't specified
        if url.path and "db" not in kwargs:
            try:
                kwargs["db"] = int(unquote(url.path).replace("/", ""))
            except (AttributeError, ValueError):
                pass

        if url.scheme == "rediss":
            kwargs["connection_class"] = SSLConnection

    return kwargs


class ConnectionPool:
    """
    Create a connection pool. ``If max_connections`` is set, then this
    object raises :py:class:`~redis.exceptions.ConnectionError` when the pool's
    limit is reached.

    By default, TCP connections are created unless ``connection_class``
    is specified. Use class:`.UnixDomainSocketConnection` for
    unix sockets.

    Any additional keyword arguments are passed to the constructor of
    ``connection_class``.
    """

    @classmethod
    def from_url(cls, url, **kwargs):
        """
        Return a connection pool configured from the given URL.

        For example::

            redis://[[username]:[password]]@localhost:6379/0
            rediss://[[username]:[password]]@localhost:6379/0
            unix://[username@]/path/to/socket.sock?db=0[&password=password]

        Three URL schemes are supported:

        - `redis://` creates a TCP socket connection. See more at:
          <https://www.iana.org/assignments/uri-schemes/prov/redis>
        - `rediss://` creates a SSL wrapped TCP socket connection. See more at:
          <https://www.iana.org/assignments/uri-schemes/prov/rediss>
        - ``unix://``: creates a Unix Domain Socket connection.

        The username, password, hostname, path and all querystring values
        are passed through urllib.parse.unquote in order to replace any
        percent-encoded values with their corresponding characters.

        There are several ways to specify a database number. The first value
        found will be used:

            1. A ``db`` querystring option, e.g. redis://localhost?db=0
            2. If using the redis:// or rediss:// schemes, the path argument
               of the url, e.g. redis://localhost/0
            3. A ``db`` keyword argument to this function.

        If none of these options are specified, the default db=0 is used.

        All querystring options are cast to their appropriate Python types.
        Boolean arguments can be specified with string values "True"/"False"
        or "Yes"/"No". Values that cannot be properly cast cause a
        ``ValueError`` to be raised. Once parsed, the querystring arguments
        and keyword arguments are passed to the ``ConnectionPool``'s
        class initializer. In the case of conflicting arguments, querystring
        arguments always win.
        """
        url_options = parse_url(url)

        if "connection_class" in kwargs:
            url_options["connection_class"] = kwargs["connection_class"]

        kwargs.update(url_options)
        return cls(**kwargs)

    def __init__(
        self, connection_class=Connection, max_connections=None, **connection_kwargs
    ):
        max_connections = max_connections or 2**31
        if not isinstance(max_connections, int) or max_connections < 0:
            raise ValueError('"max_connections" must be a positive integer')

        self.connection_class = connection_class
        self.connection_kwargs = connection_kwargs
        self.max_connections = max_connections

        # a lock to protect the critical section in _checkpid().
        # this lock is acquired when the process id changes, such as
        # after a fork. during this time, multiple threads in the child
        # process could attempt to acquire this lock. the first thread
        # to acquire the lock will reset the data structures and lock
        # object of this pool. subsequent threads acquiring this lock
        # will notice the first thread already did the work and simply
        # release the lock.
        self._fork_lock = threading.Lock()
        self.reset()

    def __repr__(self):
        return (
            f"{type(self).__name__}"
            f"<{repr(self.connection_class(**self.connection_kwargs))}>"
        )

    def reset(self):
        self._lock = threading.Lock()
        self._created_connections = 0
        self._available_connections = []
        self._in_use_connections = set()

        # this must be the last operation in this method. while reset() is
        # called when holding _fork_lock, other threads in this process
        # can call _checkpid() which compares self.pid and os.getpid() without
        # holding any lock (for performance reasons). keeping this assignment
        # as the last operation ensures that those other threads will also
        # notice a pid difference and block waiting for the first thread to
        # release _fork_lock. when each of these threads eventually acquire
        # _fork_lock, they will notice that another thread already called
        # reset() and they will immediately release _fork_lock and continue on.
        self.pid = os.getpid()

    def _checkpid(self):
        # _checkpid() attempts to keep ConnectionPool fork-safe on modern
        # systems. this is called by all ConnectionPool methods that
        # manipulate the pool's state such as get_connection() and release().
        #
        # _checkpid() determines whether the process has forked by comparing
        # the current process id to the process id saved on the ConnectionPool
        # instance. if these values are the same, _checkpid() simply returns.
        #
        # when the process ids differ, _checkpid() assumes that the process
        # has forked and that we're now running in the child process. the child
        # process cannot use the parent's file descriptors (e.g., sockets).
        # therefore, when _checkpid() sees the process id change, it calls
        # reset() in order to reinitialize the child's ConnectionPool. this
        # will cause the child to make all new connection objects.
        #
        # _checkpid() is protected by self._fork_lock to ensure that multiple
        # threads in the child process do not call reset() multiple times.
        #
        # there is an extremely small chance this could fail in the following
        # scenario:
        #   1. process A calls _checkpid() for the first time and acquires
        #      self._fork_lock.
        #   2. while holding self._fork_lock, process A forks (the fork()
        #      could happen in a different thread owned by process A)
        #   3. process B (the forked child process) inherits the
        #      ConnectionPool's state from the parent. that state includes
        #      a locked _fork_lock. process B will not be notified when
        #      process A releases the _fork_lock and will thus never be
        #      able to acquire the _fork_lock.
        #
        # to mitigate this possible deadlock, _checkpid() will only wait 5
        # seconds to acquire _fork_lock. if _fork_lock cannot be acquired in
        # that time it is assumed that the child is deadlocked and a
        # redis.ChildDeadlockedError error is raised.
        if self.pid != os.getpid():
            acquired = self._fork_lock.acquire(timeout=5)
            if not acquired:
                raise ChildDeadlockedError
            # reset() the instance for the new process if another thread
            # hasn't already done so
            try:
                if self.pid != os.getpid():
                    self.reset()
            finally:
                self._fork_lock.release()

    def get_connection(self, command_name, *keys, **options):
        "Get a connection from the pool"
        self._checkpid()
        with self._lock:
            try:
                connection = self._available_connections.pop()
            except IndexError:
                connection = self.make_connection()
            self._in_use_connections.add(connection)

        try:
            # ensure this connection is connected to Redis
            connection.connect()
            # connections that the pool provides should be ready to send
            # a command. if not, the connection was either returned to the
            # pool before all data has been read or the socket has been
            # closed. either way, reconnect and verify everything is good.
            try:
                if connection.can_read():
                    raise ConnectionError("Connection has data")
            except (ConnectionError, OSError):
                connection.disconnect()
                connection.connect()
                if connection.can_read():
                    raise ConnectionError("Connection not ready")
        except BaseException:
            # release the connection back to the pool so that we don't
            # leak it
            self.release(connection)
            raise

        return connection

    def get_encoder(self):
        "Return an encoder based on encoding settings"
        kwargs = self.connection_kwargs
        return Encoder(
            encoding=kwargs.get("encoding", "utf-8"),
            encoding_errors=kwargs.get("encoding_errors", "strict"),
            decode_responses=kwargs.get("decode_responses", False),
        )

    def make_connection(self):
        "Create a new connection"
        if self._created_connections >= self.max_connections:
            raise ConnectionError("Too many connections")
        self._created_connections += 1
        return self.connection_class(**self.connection_kwargs)

    def release(self, connection):
        "Releases the connection back to the pool"
        self._checkpid()
        with self._lock:
            try:
                self._in_use_connections.remove(connection)
            except KeyError:
                # Gracefully fail when a connection is returned to this pool
                # that the pool doesn't actually own
                pass

            if self.owns_connection(connection):
                self._available_connections.append(connection)
            else:
                # pool doesn't own this connection. do not add it back
                # to the pool and decrement the count so that another
                # connection can take its place if needed
                self._created_connections -= 1
                connection.disconnect()
                return

    def owns_connection(self, connection):
        return connection.pid == self.pid

    def disconnect(self, inuse_connections=True):
        """
        Disconnects connections in the pool

        If ``inuse_connections`` is True, disconnect connections that are
        current in use, potentially by other threads. Otherwise only disconnect
        connections that are idle in the pool.
        """
        self._checkpid()
        with self._lock:
            if inuse_connections:
                connections = chain(
                    self._available_connections, self._in_use_connections
                )
            else:
                connections = self._available_connections

            for connection in connections:
                connection.disconnect()

    def set_retry(self, retry: "Retry") -> None:
        self.connection_kwargs.update({"retry": retry})
        for conn in self._available_connections:
            conn.retry = retry
        for conn in self._in_use_connections:
            conn.retry = retry


class BlockingConnectionPool(ConnectionPool):
    """
    Thread-safe blocking connection pool::

        >>> from redis.client import Redis
        >>> client = Redis(connection_pool=BlockingConnectionPool())

    It performs the same function as the default
    :py:class:`~redis.ConnectionPool` implementation, in that,
    it maintains a pool of reusable connections that can be shared by
    multiple redis clients (safely across threads if required).

    The difference is that, in the event that a client tries to get a
    connection from the pool when all of connections are in use, rather than
    raising a :py:class:`~redis.ConnectionError` (as the default
    :py:class:`~redis.ConnectionPool` implementation does), it
    makes the client wait ("blocks") for a specified number of seconds until
    a connection becomes available.

    Use ``max_connections`` to increase / decrease the pool size::

        >>> pool = BlockingConnectionPool(max_connections=10)

    Use ``timeout`` to tell it either how many seconds to wait for a connection
    to become available, or to block forever:

        >>> # Block forever.
        >>> pool = BlockingConnectionPool(timeout=None)

        >>> # Raise a ``ConnectionError`` after five seconds if a connection is
        >>> # not available.
        >>> pool = BlockingConnectionPool(timeout=5)
    """

    def __init__(
        self,
        max_connections=50,
        timeout=20,
        connection_class=Connection,
        queue_class=LifoQueue,
        **connection_kwargs,
    ):

        self.queue_class = queue_class
        self.timeout = timeout
        super().__init__(
            connection_class=connection_class,
            max_connections=max_connections,
            **connection_kwargs,
        )

    def reset(self):
        # Create and fill up a thread safe queue with ``None`` values.
        self.pool = self.queue_class(self.max_connections)
        while True:
            try:
                self.pool.put_nowait(None)
            except Full:
                break

        # Keep a list of actual connection instances so that we can
        # disconnect them later.
        self._connections = []

        # this must be the last operation in this method. while reset() is
        # called when holding _fork_lock, other threads in this process
        # can call _checkpid() which compares self.pid and os.getpid() without
        # holding any lock (for performance reasons). keeping this assignment
        # as the last operation ensures that those other threads will also
        # notice a pid difference and block waiting for the first thread to
        # release _fork_lock. when each of these threads eventually acquire
        # _fork_lock, they will notice that another thread already called
        # reset() and they will immediately release _fork_lock and continue on.
        self.pid = os.getpid()

    def make_connection(self):
        "Make a fresh connection."
        connection = self.connection_class(**self.connection_kwargs)
        self._connections.append(connection)
        return connection

    def get_connection(self, command_name, *keys, **options):
        """
        Get a connection, blocking for ``self.timeout`` until a connection
        is available from the pool.

        If the connection returned is ``None`` then creates a new connection.
        Because we use a last-in first-out queue, the existing connections
        (having been returned to the pool after the initial ``None`` values
        were added) will be returned before ``None`` values. This means we only
        create new connections when we need to, i.e.: the actual number of
        connections will only increase in response to demand.
        """
        # Make sure we haven't changed process.
        self._checkpid()

        # Try and get a connection from the pool. If one isn't available within
        # self.timeout then raise a ``ConnectionError``.
        connection = None
        try:
            connection = self.pool.get(block=True, timeout=self.timeout)
        except Empty:
            # Note that this is not caught by the redis client and will be
            # raised unless handled by application code. If you want never to
            raise ConnectionError("No connection available.")

        # If the ``connection`` is actually ``None`` then that's a cue to make
        # a new connection to add to the pool.
        if connection is None:
            connection = self.make_connection()

        try:
            # ensure this connection is connected to Redis
            connection.connect()
            # connections that the pool provides should be ready to send
            # a command. if not, the connection was either returned to the
            # pool before all data has been read or the socket has been
            # closed. either way, reconnect and verify everything is good.
            try:
                if connection.can_read():
                    raise ConnectionError("Connection has data")
            except (ConnectionError, OSError):
                connection.disconnect()
                connection.connect()
                if connection.can_read():
                    raise ConnectionError("Connection not ready")
        except BaseException:
            # release the connection back to the pool so that we don't leak it
            self.release(connection)
            raise

        return connection

    def release(self, connection):
        "Releases the connection back to the pool."
        # Make sure we haven't changed process.
        self._checkpid()
        if not self.owns_connection(connection):
            # pool doesn't own this connection. do not add it back
            # to the pool. instead add a None value which is a placeholder
            # that will cause the pool to recreate the connection if
            # its needed.
            connection.disconnect()
            self.pool.put_nowait(None)
            return

        # Put the connection back into the pool.
        try:
            self.pool.put_nowait(connection)
        except Full:
            # perhaps the pool has been reset() after a fork? regardless,
            # we don't want this connection
            pass

    def disconnect(self):
        "Disconnects all connections in the pool."
        self._checkpid()
        for connection in self._connections:
            connection.disconnect()